1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine for controlling operation timings of an intake valve or an exhaust valve of the internal combustion engine.
2. Description of the Related Art
Conventionally, a valve timing control apparatus for an internal combustion engine changes a phase angle of a camshaft with respect to a crankshaft of the internal combustion engine, thereby changing timings for opening and closing an intake valve or an exhaust valve. This valve timing control apparatus is equipped with a crank angle sensor for outputting a crank angle signal when the crankshaft is at a reference rotational position, and a cam angle sensor for outputting a cam angle signal when the camshaft is at a reference rotational position. The valve timing control apparatus detects an actual phase angle of the camshaft based on detection signals from the crank angle sensor and the cam angle sensor, and performs phase angle feedback control such that the actual phase angle coincides with a target phase angle set based on an operational state of the internal combustion engine.
A variable camshaft phase mechanism, which is supplied with a hydraulic pressure controlled by a hydraulic pressure control solenoid valve, changes the phase angle of the camshaft with respect to the crankshaft.
The hydraulic pressure control solenoid valve, which is designed as a duty solenoid valve, controls the duty ratio of the voltage supplied to a solenoid to control the value of a current flowing therethrough, and selectively supplies a hydraulic pressure to an advancement chamber or a retardation chamber of the variable camshaft phase mechanism, so the camshaft is shifted to an advancement side or a retardation side. When the duty ratio assumes a holding duty value in the neighborhood of a median, the hydraulic pressure control solenoid valve simultaneously closes the advancement chamber and the retardation chamber, and controls the position thereof to a neutral position for simultaneously shutting off the supply of hydraulic pressures to the advancement chamber and the retardation chamber, so the phase angle of the camshaft is held.
In order to compensate for variations in the holding duty value for holding the hydraulic pressure control solenoid valve at the neutral position, which result from a tolerance, aged deterioration, and the like of the hydraulic pressure control solenoid valve, it is known to learn the holding duty value or store the learning value thereof into a backup RAM.
It is also known to use a fixed value stored in advance in a ROM as an initial value when the holding duty value is not learned at all, or when the learning value is lost by, for example, turning a battery OFF (disconnecting a terminal of the battery).
As a matter of course, however, owing to a certain variation width of the tolerance and aged deterioration, the fixed value of the holding duty set as described above may not coincide with the learning value for compensating for the tolerance and aged deterioration. In the case of such a deviation, therefore, when the fixed value of the holding duty value is used as the initial value, for example, during the battery being turned OFF, the actual position of the hydraulic pressure control solenoid valve in a holding state thereof deviates from the original neutral position. In consequence, the controllability of subsequent cam phase control also deteriorates.
Especially in a case where this deviation occurs on the advancement side and the target phase angle is set on the advancement side where the amount of valve overlap between the intake valve and the exhaust valve is intrinsically large, it is also known that the amount of valve overlap becomes excessively large, that the amount of internal EGR thereby becomes excessively large, with the result that a deterioration in combustibility may be caused.
Thus, this valve timing control apparatus sets the learning value of the holding duty as an initial value of an integral term of feedback control, and limits the target phase angle in a case where the holding duty has not been learned yet (e.g., see JP 2001-234765 A).
In this valve timing control apparatus for the internal combustion engine, however, the holding duty fluctuates due to changes in the resistance value of the hydraulic pressure control solenoid coil, which result from changes in oil temperature, or changes in battery voltage. Therefore, the actual value of the holding duty value deviates from the learning value thereof when the temperature of the hydraulic pressure control solenoid coil and the battery voltage in learning the holding duty are different respectively from the temperature and the voltage in setting the learning value of the holding duty as the initial value of the integral term at the beginning of phase angle feedback control.
In such a case, the actual position of the hydraulic pressure control solenoid valve in the holding state thereof deviates from the original neutral position when the learning value of the holding duty is set as the initial value of the integral term at the beginning of phase angle feedback control following the start of the internal combustion engine. Especially in a case where this deviation arises on the advancement side and the target phase angle is set on the advancement side where the amount of valve overlap between the intake valve and the exhaust valve is intrinsically large, the amount of valve overlap becomes excessively large. In consequence, the amount of internal EGR (amount of exhaust gas recirculation) becomes excessively large, so a deterioration in startability of the internal combustion engine is caused.
The target phase angle is limited in the case where the value of the holding duty has not been learned yet, so there is a limit to the control on the advancement side. In an internal combustion engine equipped with a valve timing control apparatus for changing timings for opening and closing an intake valve, the timing for closing the intake valve is retarded when the timings for opening/closing the intake valve are shifted too much to the retardation side in starting the internal combustion engine. Thus, the mixture sucked into a combustion chamber flows back into an intake pipe.
When the sucked mixture flows back into the intake pipe at the time of cranking, which is associated with an extremely low rotational speed of the internal combustion engine, a decrease in actual compression ratio is caused, so it becomes difficult to start the internal combustion engine. In particular, there is a problem in that the mixture is not sufficiently compressed despite cranking and hence a further deterioration in startability is caused when the internal combustion engine is at a low temperature, namely, when the mixture is small in volume.